1. Field of Invention
This invention relates to thermal fluid ejection systems.
2. Description of Related Art
Thermal fluid ejection systems, such as, for example, thermal ink jet printers, use thermal energy selectively produced by resistors located in fluid filled channels or chambers near channel-terminating nozzles. Firing signals are applied to the resistors through associated drive circuitry to momentarily vaporize the fluid and form bubbles on demand. Each temporary bubble expels a fluid droplet and propels it towards a receiving medium. The fluid ejector head is usually sealingly attached to one or more fluid supply containers and the combined fluid ejector and container form a cartridge assembly which is, in various exemplary embodiments, reciprocated to eject one swath of fluid at a time on a stationarily-held receiving medium such as paper.
A typical thermal fluid ejector head consists of an array of resistive heaters, each of which is located within a channel that is filled with fluid during operation. When a given heater is pulsed, that heater nucleates a fluid vapor bubble, which grows and propels a droplet of fluid out of the channel and onto the receiving medium. The size of the ejected fluid droplet is particularly dependent upon the maximum size of the bubble, which is in turn partially dependent upon the amount of thermal energy transferred to the fluid and upon the heater size.
For example, in a standard thermal ink jet printhead, the heaters are nominally uniform. In such a fluid ejector head, substantially the entire heater surface reaches the bubble nucleating temperature at the same time when the appropriate magnitude voltage pulse is applied. Once the bubble nucleates and grows, little thermal energy is usually transferred into the fluid because of the poor thermal conductivity of the fluid vapor bubble. Thus, for standard heater construction, in which bubble growth occurs substantially over the entire heater surface at the same time, continued power dissipation in the heater is not effective in forming larger fluid drops.
During ejection operations, the droplet ejected from the fluid ejector head to the receiving member forms a spot of fluid. In a thermal ink jet printer, the ejected fluid is ink that forms a spot as a part of a desired image. The human eye is very sensitive to changes in spot size, especially when shaded areas and graphics are being produced, especially for color printing. Therefore, uniformity of spot size of a large number of droplets is crucial to maintaining image quality in thermal ink jet printing. If the volume of ejected droplets varies greatly within a single image, the lack of uniformity in droplet volume will noticeably affect the size of the ink spots forming the image and detract from the quality of the image. Similarly, if volumes of droplets ejected from the printhead differ during subsequent printings of the same image, then printing consistency cannot be maintained. Alternatively, by controllably varying the droplet size, continuous tone and cluster dot half-tone printing can be obtained.
Accordingly, because print quality in thermal ink jet printing highly depends on the controllability of the printhead, i.e., through the use of heating elements in specific configurations to vary controllably the size of drops that are recorded on the medium, a device which can better control a range of sizes of drops of ink which are ejected is desired. Similarly, in general, the ability to control the drop size over a range of sizes is desirable in any fluid ejection system, not only thermal ink jet printers.
A heater element of substantially constant resistance and cross-section across its length and width tends to have a limited range of bubble sizes, and hence drop sizes, regardless of applied pulse width and/or voltage. However, a design in which heater elements are controllably nonuniform can nucleate at a relatively low level of energy a smaller bubble size over a segment of the heater element where the power density is highest. Successively larger bubbles, and hence larger ejected droplets, can be generated when the pulse width and/or voltage is increased to the level that the power density is raised sufficiently in other segments of the heater element.
This invention will describe various configurations of segmented heaters, i.e., for example, both where the different heater segments having different resistances are electrically connected in series with each other, and in which the heater segments are electrically connected in parallel with each other. In one aspect, configurations are described in which the bubble nucleation occurs sequentially along the length of the heater, with the first bubble being closest to the edge of the device, as would be appropriate in a side-shooting printhead. In another aspect, configurations are also described in which bubble nucleation occurs first in the center of the heater element array and proceeds substantially radially outward, as would be appropriate for a roof-shooting printhead.
Conceptually, the simplest configuration of the segmented heaters is the one in which the heater segments of different resistance are connected end to end in series. For the case where the different resistances are achieved by different doping levels in the heater element, one problem associated with serially-connected heater segments is that dopants diffuse directly from one heater segment type into another heater segment type, causing the heater segments, sometimes each with various doping levels, to degrade.
Yet another problem associated with serially-connected heater segments is that the heater segments are heavily doped in order to have sufficiently low resistance that they can be pulsed to eject droplets at voltages of less than 50 volts. Accordingly, for the case with polysilicon heater elements, heavily doped heater segments can be rough in texture, which results in unwanted bubble nucleation sites.
One problem associated with a parallely-connected heater segments is that there may be very limited space for the electrical connection of the leads to the heater segments. In addition, some high resolution fluid ejectors have small channel spacings that cause some parallel connections to be impractical.
Accordingly, this invention provides apparatus and systems that have improved parallely-connected heater segments electrically connected so that there are lower print voltage requirements, thus reducing the cost of the power supplies.
This invention separately provides apparatus and systems that allow the spacing between heater segments to be varied to modify and/or control fluid ejection characteristics.
This invention separately provides apparatus and systems that provide improved controllability in each heater segment.
This invention separately provides apparatus and systems that have heater segments with various lengths and/or widths, and by different doping levels, in order to produce the different power densities in each heater segment.
This invention separately provides apparatus and systems that have parallely-connected heater segments with reduced heater segment degradation.
This invention separately provides apparatus and systems that reduces pathways for dopants to diffuse from one heater segment type into another during high temperature processing.
This invention separately provides apparatus and systems that provide heater segments having lower doping levels and/or lower resistance.
This invention separately provides apparatus and systems that relax space limiting factors in fluid ejector devices.
This invention separately provides apparatus and systems that allow high voltage and low voltage lines to be spaced further apart.
This invention separately provides apparatus and systems that provide heater segments having power density variations having increased radial symmetry.
This invention separately provides apparatus and systems having heater segments that are able to fire small droplets of fluid.
In various exemplary embodiments of the apparatus and systems according to this invention, area and power level dissipation of each heater segment in the ink jet printhead are chosen such that different sized drops of fluid are ejected depending on the pulse voltage and/or pulse width used. As the pulse voltage and/or pulse width applied to a particular channel is increased, more heating segments in that ejection channel nucleate bubbles and produce larger drops. As a result, drop volume and spot size of the ejected fluid can be increased as pulse voltage and/or pulse width increase.
In various other exemplary embodiments, the heater segments are arranged into a two-dimensional array. Each segment has a different power density. The segmented arrays are arranged with the highest power density elements located near the center of the array. Other heater segments having lower power densities are located progressively further from the center heater segments. As the heater segment power voltage and/or pulse width is increased, successively more heater elements nucleate bubbles and produce larger drops of ejected fluid.
In particular, in various exemplary embodiments, the nucleating bubbles spread from the center outward in a radial fashion. In various exemplary embodiments, the heater segments have a wide variety of geometrical shapes which enable a more radial variation in the power density.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description and various exemplary embodiments of the apparatus and systems according to this invention.